chrisballinger/poly

A general-purpose library to help represent situations where the type of a value is one of a set of types.

Poly

Poly is a small library to provide an alternative to rolling your own type-erasure when a value has one of a small set of Types. The Poly library contains the Types Poly1, Poly2, Poly3, etc. for representing increasingly larger pools of possible Types. Poly2 is isomorphic to Either (a common generic functional programming Type).

Dev Environment

Prerequisites

1. Swift 4.2+
2. Swift Package Manager 5.0 OR Cocoapods

CocoaPods

To use this framework in your project via Cocoapods instead of Swift Package Manager, add the following dependency to your Podfile.

	pod 'Poly', :git => 'https://github.com/mattpolzin/Poly.git'

Xcode project

To create an Xcode project for Poly, run swift package generate-xcodeproj

Usage

Usage will be explained by way of an example. Suppose you have some code with three different Types: Dog, Cat, and Rat. You also have a protocol, Animal, that they all belong to.

If you need to store animals of all three Types in one place (maybe an array), that looks like:

let dog = Dog()
let cat = Cat()
let rat = Rat()

let animals: [Poly3<Dog, Cat, Rat>] = [
  .init(dog),
  .init(cat),
  .init(rat)
]

To access all animals of a certain type, you can use subscripting like:

let dogs = animals[Dog.self]
let cats = animals[Cat.self]
let rats = animals[Rat.self]

You can get the Dog, Cat, or Rat value back out again, but you won't get any guarantees of which Type is being stored in a given Poly:

let animal = Poly3<Dog, Cat, Rat>(Dog())

let maybeDog: Dog? = animal.a
let maybeCat: Cat? = animal.b
let maybeRat: Rat? = animal.c

Or use the subscript operator to make accessing one of the possible values of a Poly a bit more intuitive:

let maybeDog2 = animal[Dog.self]
let maybeCat2 = animal[Cat.self]
let maybeRat2 = animal[Rat.self]

Or switch over the possible values:

switch animal {
  case .a(let dog):
    print(dog)
  case .b(let cat):
    print(cat)
  case .c(let rat):
    print(rat)
}

You might consider making a typealias to make your life easier:

typealias AnyAnimal = Poly3<Dog, Cat, Rat>

You also might find it worthwhile to go the extra mile and add Animal conformance to Poly<Dog, Cat, Rat>:

protocol Animal {
  var speak: String { get }
}

extension Poly3: Animal where A == Dog, B == Cat, C == Rat {
  var speak: String {
    switch self {
      case .a(let animal as Animal),
           .b(let animal as Animal),
           .c(let animal as Animal):
          return animal.speak
    }
  }
}

So now you can take the array of animals from the first example above and:

let animalSounds = animals.map { $0.speak }

Codable

All of the Polytypes are Encodable/Decodable when every generic on which they are specialized is Ecodable/Decodable. This works by attempting to encode or decode each type and using the first successfuly attempt. That means the behavior only works as expected if all of the types have different encoding/decoding requirements or at least types are ordered with more restrictive rules coming first.

For example, given the following type

struct Type1: Decodable {
    let x: Poly2<Double, Int> 
}

an integer value will never be decoded as an Int because Double is capable of decoding all Int values.

You can fix this by swapping the order to Poly2<Int, Double>. Now Poly will attempt to decode an Int and only attempt to decode a Double if an Int was not found.